1. Field
The present invention relates generally to communications systems, and more specifically to systems and methods for optimizing soft handoff and system access parameters in telecommunications systems.
2. Background
Cellular telecommunications systems are characterized by a plurality of mobile units (e.g. cellular telephones) in communication with one or more base stations. Signals transmitted by a mobile unit are received by a base station and often relayed to a mobile switching center (MSC). The MSC in turn routes the signals to a public switched telephone network (PSTN) or to another mobile unit. Similarly, a signal may be transmitted from the PSTN to the mobile unit via the base station and the MSC.
Each base station covers a “cell” within which a mobile unit may communicate. The cell covers a limited geographic area, wherein calls from mobile units are routed to and from a telecommunications network via the MSC. The coverage area of a typical cellular telecommunications system may be divided into several cells. Each cell may also be divided into several sectors. Different communications resources are often allocated to each cell or sector to maximize communications system resources. When a mobile unit moves from a first cell to a second cell, or from a first sector to a second sector, a handoff has to be performed to assign new system resources associated with the second cell or sector.
A handoff involves executing a set of negotiations between the mobile unit and one or more governing base stations and/or MSCs. Handoff improves the performance of the system at the expense of more system resource. Efficient and timely handoff procedures are becoming increasingly important as smaller cells and/or sectors are deployed to meet demands for increased communications system capacity. Use of smaller cells and/or sectors increases the number of boundary crossings and resource assignments, thereby increasing the need for adaptive, efficient, fast, and cost-effective handoff procedures.
Handoff can be classified as hard handoff or soft handoff. Hard handoff procedures are used to transfer an ongoing call between adjacent cells or sectors, having different frequency assignments, having different radio configurations as in the case of third generation wireless systems (3G), having different frame offsets, or even between systems such as handoff between code division multiple access (CDMA) and analog (AMPS). In a hard handoff, a first link with a first cell is broken and then a second link is established. In a soft handoff, a first link is maintained until a second link is established. Thus, there is a time during which the first link and second link are maintained simultaneously. In either the case, a large delay between the dropping of the first link and the establishment of the second link may result in unacceptable communications service quality.
Access handoff is another feature of 3G systems. Due to the rapid change in the dynamics of the radio frequency (RF) channel, the control (paging) channel may not be in soft handoff when the traffic channel is assigned, and the mobile station may not be monitoring the best cell when it receives a page. Consequently, the performance of the phone while operating in system access state is vulnerable. To enhance the performance of the system while the mobile station is in the system-access state, some techniques have been proposed. These techniques include access entry handoff, channel assignment into soft handoff, access handoff, and access probe handoff.
In processing a handoff, a mobile unit uses various static handoff parameters, which may have been sent over the air and stored by the mobile unit. One problem with using static handoff parameters is that the mobile unit has to use the same static parameters for all geographical areas, irrespective of terrain, morphology, traffic density of cell sites and/or sectors, and other land characteristics. Consequently, handoffs based on static handoff parameters are not adaptable to various geographical locations, may consume more system resources, and may result in poor communications service performance.
There is a need, therefore, for an adaptive, fast, efficient and cost-effective method and system for facilitating reliable system access and soft handoff in a cellular telecommunications system with optimized parameters based on location information.